High voltage lithium ion positive electrode material

ABSTRACT

A positive electrode material having a nominal stoichiometry Li 1+y/2 Co 1−x−y−z−d Si z Fe x M y M′ d (PO 4 ) 1+y/2  where M is a trivalent cation selected from at least one of Cr, Ti, Al, Mn, Ni, V, Sc, La and/or Ga, M′ is a divalent cation selected from at least one of Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y is within a range of 0&lt;y≦0.10 and x is within a range of 0≦x≦0.2. The use of double compositional modification to LiCoPO 4  increases the discharge capacity from ˜100 mAh/g to about 130 mAh/g while retaining the discharge capacity retention of the singly Fe-substituted LiCoPO 4 . Additional compositional modification to include Si increases the cycle life and greatly improved the coulombic efficiency to between 97-100% at a C/3 cycle rate,

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 61/911,700 filed on 4 Dec. 2013, the complete disclosures of which, in their entirety, are herein incorporated by reference.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensed by or for the United States Government.

FIELD OF INTEREST

The present invention relates in general to a positive electrode material, and in particular to a high voltage lithium ion positive electrode material.

BACKGROUND

LiFePO₄[1] is a Li-ion positive electrode material that has been favored for its strong abuse tolerance which in turn has been attributed to the nature of the oxygen bonding in the phosphate group [2]. Also, it is desirable to harness the abuse tolerance of a phosphate based cathode in a material that stores more energy. One possibility is to look to higher voltage olivines such as LiMnPO₄ [1] 4.1 V, LiCoPO₄ [3] 4.8 V or LiNiPO₄ [4] 5.1V since the stored energy is proportional to the voltage. LiCoPO₄ in particular has the potential to increase energy ˜40% compared to LiFePO₄. Furthermore, its electronic structure is more favorable than LiMnPO₄ and LiNiPO₄ in terms of polaronic conductivity and ability to form polarons, respectively [5].

However, and even though initial research on LiCoPO₄ led to improvements in the rate of discharge, capacity fade has blocked further progress [6-10]. As such, an improved Li-ion positive electrode material with a high discharge capacity and low capacity fade would be desirable.

SUMMARY

A Li-ion positive electrode material is provided. The material has a nominal stoichiometry of Li_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) where M is a trivalent cation such as Cr, Ti, Al, Mn, Ni, V, Sc, La and/or Ga, M′ is a divalent cation such as Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y is within a range of 0<y≦0.10, x is within a range of 0≦x≦0.2, z is within a range of 0≦z≦0.1 and d is within a range of 0≦d≦0.20. In some instances, d is within the range of 0≦d≦0.10, and preferably within the range of 0≦d≦0.05. The Li-ion positive electrode material has an initial capacity of at least 120 mAh/g and a discharge capacity of at least 100 mAh/g after 500 cycles.

In some instances, the positive electrode material has a composition where y is within the range of 0.02≦y≦0.08, x is within the range of 0.05≦x≦0.15 and M=Cr or Ti. In other instances, y is within the range of 0.04≦y≦0.06, x is within the range of 0.08≦x≦0.12 amd M=Cr or Ti, In still other instances, y=0.05, x=0.10 and M=Cr or Ti.

In one embodiment, z and d are equal to zero, and the Li-ion positive electrode material has a nominal stoichiometry of Li_(1.025)Co_(0.85)Fe_(0.1)Cr_(0.05)(PO₄)_(1.025), an initial capacity of at least 125 mAh/g and a discharge capacity of at least 105 mAh/g after 500 cycles.

In another embodiment, d is equal to zero, z is not equal to zero, the positive electrode material contains Si and the Li-ion positive electrode material has a nominal stoichiometry of Li_(1+y/2)Co_(1−x−y−z)Si_(z)Fe_(x)M_(y)(PO₄)_(1+y/2) where x and y have the values given above and z is within a range of 0<z≦0.1, preferably within a range of 0<z≦0.05, and more preferably within a range of 0<z≦0.02. In some instances z=0.01. Also, the addition of Si improves the coulombic efficiency of the material and in some instances the coulombic efficiency is between 97-100% at a C/3 cycle rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical plot of discharge capacity as a function of cycle number for samples of composition Li_(1+y/2)Co_(1−x−y)Fe_(x)M_(y)(PO₄)_(1+y/2) with M=Cr, Ti, Al, and Ga, and where 10Ti10Fe represents Li1.05Co_(0.80)Fe_(0.10)Ti_(0.10)(PO₄)_(1.05), 5Ti10Fe represents Li_(0.25)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025), etc.;

FIG. 2 is a graphical plot of discharge capacity as a function of cycle number for materials according to an embodiment of the present invention;

FIG. 3 is a graphical illustration of long term discharge capacity as a function of cycle number for materials according to an embodiment of the present invention;

FIG. 4 is a graphical plot of X-ray powder diffraction patterns for Li_(1.05)Co_(0.8)Fe_(0.10)Cr_(0.10)(PO₄)_(1.05) (Top) and Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) (Bottom), with the peaks labeled with the Miller indices of the phospho-olivine structure. Pnma spacegroup.

FIG. 5 is a graphical plot of X-ray powder diffraction patterns of Li_(1.05)Co_(0.8)Fe_(0.10)(PO₄)_(1.05) (Top), Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) (Middle) and Li_(1.0125)Co_(0.875)Fe_(0.10)Cr_(0.025)(PO₄)_(1.0125). (Bottom), with the peaks labeled with the Miller indices of the phospho-olivine structure, pima spacegroup;

FIG. 6 is a graphical plot of voltage versus discharge capacity of Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) as a faction of rate;

FIG. 7 is a graphical plot of capacity and coulombic efficiency of (A) Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025); versus (B) Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025);

FIG. 8 is a graphical plot of capacity and coulombic efficiency of: (A) Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025); versus (B) Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025); and

FIG. 10 is a graphical plot of an X-ray powder diffraction pattern of Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025).

DETAILED DESCRIPTION

An improved Li-ion positive electrode material with an initial capacity of at least 120 mAh/g and a discharge capacity of at least 100 mAh/g after 500 cycles is provided. In some instances, the improved Li-ion positive electrode material has initial capacity of at least 125 mAh/g and a discharge capacity of at least 105 mAh/g after 500 cycles. In addition, the material can contain Si which provides an coulombic efficiency to between 97-100% at a C/3 cycle rate,

it is appreciated that the improved capacity and dramatically reduced capacity fade is striking relative to LiCoPO₄. The use of a double compositional modification increases the discharge capacity from 100 mAh/g to about 130 mAh/g in the most favorable cases for Ti and Fe or Cr and Fe modification of LiCoPO₄, while retaining the discharge capacity retention of the singly Fe-substituted LiCoPO₄. Additional compositional modification to include Si increases the cycle life and greatly improves the coulombic efficiency to between 97-100% at a C/3 cycle rate.

The material has a nominal stoichiometry of Li_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) where M is a trivalent cation such as Cr, Ti, Al, Ni, V, Sc, La and/or Ga, M′ is a covalent cation such as Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y is within a range of 0<y≦0.10, x is within a range of 0≦x≦0.2, z is within the range of 0≦z≦0.1 and d is within the range of 0≦d≦0.20. In some instances, y is within the range of 0.02≦y≦0.08, x is within the range of 0.05≦x≦0.15 and M=Cr or Ti. In other instances, y is within the range of 0.04≦y≦0.06, x is within the range of 0.08≦x≦0.12 and M=Cr or Ti. In still other instances, y=0.05, x=0.10 and M=Cr or Ti.

The positive electrode material can also contain Si and the Li-ion positive electrode material can have a nominal stoichiometry of Li_(1+y/2)Co_(1−x−y−z)Si_(z)Fe_(x)M_(y)(PO₄)_(1+y/2) where x and y have the values given above and z is within a range of 0<z≦0.1, preferably within a range of 0<z≦0.05, and more preferably within a range of 0<z≦0.02. In some instances z=0.01.

In order to better teach the invention but not limit its scope in any way, a solid state synthesis method for making the Li_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) material and one or more examples of the inventive material are discussed below.

Samples of Li_(1+y/2)Co_(1−x−y−Fe) _(x)M_(y)(PO₄)_(1+y/2) with M=Cr Ti, Al and/or Ga, 0<y≦0.10 and 0≦x≦0.2 were prepared via a solid state route. Stoichiometric amounts of Co(OH)₂, LiH₂PO₄, Cr₂O₃, TiO₂, Al(OH)₃, Ga₂O₃, FeC₂O₄.2H₂O and/or acetylene black (5 wt. % of final product) were ball-milled for 90 minutes. The mixture was then heated at a heating rate of 10° C. min⁻¹ to 700° C. under flowing N₂ and then the reactant mixture was held at this temperature for 12 h. Samples of Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)M_(0.05)(PO₄)_(1.025) with M=Cr or Ti were prepared via the same method using Si(OOCCH₃)₄ as the Si source.

The resulting crystalline phase(s) were identified using X-ray powder diffraction, X-ray data were collected using a Rigaku Ultima III diffractometer, Lattice constants were calculated from peak positions using Rietveld refinement of the pattern collected in a parallel beam geometry. For electrochemical testing, a composite electrode was fabricated by a slurry coating method. Using N-methylpyrrolidone (NMP) as solvent, a slurry was used to coat an Al foil substrate to produce a composite electrode of ca. 80 wt. % active material, 10 wt. % polyvinylidene fluoride (PVDF), 8 wt. % super-P carbon and 2 wt. % conductive carbon nanotuhe composite (CheapTubes.com). The electrode film was cut into small discs with an area of 0.97 cm² and dried under an infrared lamp in air before use. In a dry room (Dew point <−50° C.), Li/active coin cells (Hohsen CR2032) were assembled using 3 layers of Celgard® 2400 as the separator and a 1.0 molal LiPF₆ solution in a 3:7 (wt. %) mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) electrolyte with 1 wt. % HFiP. Also, 100-150 μL of electrolyte was used per coin cell and electrochemical testing was performed using a Maccor Series 4000 tester. For calculation of C-rate, a capacity of ˜150 mA h g⁻¹ was assumed.

Substitution of elements in addition to Fe for Co, including Cr, Ti, Al and Ga, increased the discharge capacity of Fe-substituted LiCoPO₄ while maintaining long cycle life. Not being bound by theory, a nominal stoichiometry, Li_(1+y/2)Co_(1−x−y)Fe_(x)M_(y)(PO₄)_(1+y/2) (M=Cr, Ti, Al and/or Ga) that favors substitution on the Co site and cation vacancies exhibited favorable performance since substitution on the Li site blocks Li-ion conduction. In addition, the above stoichiometry favors the presence of cation vacancies that might enhance Li-ion conductivity.

Turning now to FIG. 1, capacity as a function of cycle for a number of different compositions are shown. The Ti,Fe and Cr,Fe double modifications exhibited the most favorable discharge capacity and cycle life. Furthermore, a composition level was discovered to be 0.10 for Fe and 0.05 for Ti or Cr. All samples exhibited superior cycle life compared to LiCoPO₄ and increased discharge capacity relative to singly Fe-substituted LiCoPO₄—labeled 20Fe(ARL) in the figure and corresponds to nominal stoichiometry of LiCo_(0.8)Fe_(0.2)PO₄).

FIG. 2 demonstrates the effect of Ti,Fe and Cr,Fe double compositional modifications on the capacity fade at the optimized dopant level of 0.10 Fe and 0.05 Ti or Cr, All samples including the un-substituted LiCoPO₄ control sample were cycled between 3.5 and 5 V using 1 M LiPF₆ in 3:7 by weight ethylene carbonate: ethyl methyl carbonate electrolyte containing 1 wt. % HFiP electrolyte additive, Cells were charged using a C/3 constant current to 5 V followed by a constant voltage of 5 V until the current was less than C/15. It is appreciated that the terms “C”, “C/3”, “C/15”, etc., refer to the C-rate used by the battery industry to scale the charge and discharge current of a battery, For example, a 1,000 mAh battery discharged at a IC rate ideally provides a current of 1,000 mA for one hour, whereas a C/2 discharge rate fur 1,000 mAh battery ideally provides a current of 500 mA for two hours.

As shown in FIG. 2, the un-substituted LiCoPO₄ control sample exhibited severe capacity fade. However, the Fe-substitution into LiCoPO₄ (nominal LiCo_(0.8)Fe_(0.2)PO₄) had a significant reduction in capacity fade relative to the un-substituted LiCoPO₄. Also, increasing the discharge capacity by further composition modification with another element in addition to Fe is shown. For example, the Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) sample showed better capacity retention with cycling despite having a slightly smaller initial discharge capacity than Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025).

FIG. 3 compares the long term cycling of composition Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) relative to LiCo_(0.8)Fe_(0.2)PO₄. The Fe-substituted LiCoPO₄ retained about 80% of its capacity at the 500th cycle (89 of 108 mAh g⁻¹ initial capacity). The Cr and Fe-doubly substituted LiCoPO₄ had an initial capacity higher than Fe substituted LiCoPO₄, but also retained about 80% of capacity at the 500^(th) cycle (107 of 131 mAh g⁻¹ initial capacity).

FIG. 4 is a comparison the XRD patterns of Li_(1+y/2)Co_(0.90·y)Fe_(0.10)Cr_(y)(PO₄)_(1+y/2) where y=0.05 and 0.10. As observed in the lower XRD pattern, Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) forms a phospho-olivine structure without other XRD detectable phases, The pattern can be indexed to the phospho-olivine structure confirming that a single phase was present. From Rietveld analysis of the X-ray diffraction data, the unit cell volume of Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) was determined to be 282.9 Å³. Also, the lattice parameters are listed in Table 1 below with comparison to LiCoPO₄ [11] and LiCo_(0.9)Fe_(0.1)PO₄ [12]. The unit cell volumes of LiCoPO₄ and LiCo_(0.9)Fe_(0.1)PO₄ are 284.3 Å³ and 285.1 Å³, respectively. This decrease in unit cell volume fur Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) is consistent with the substitution of the considerably smaller 6-coordinate Cr³⁺ (effective ionic radius=0.615 Å [13]) for Co²⁺ (effective ionic radius=0.745 Å [13]) and Fe²⁺ (effective ionic radius=0.780 Å[13]), and the presence of cation vacancies which are required to maintain charge neutrality. The decrease in unit cell volume for the Cr,Fe substituted LiCoPO₄ results from a decrease in the a and b lattice parameters with little change in the c parameter.

Not being bound by theory, improved discharge electrochemical performance may likely result from increased electronic and/or ionic conductivity. The Li-ion conductivity is, of course, a function of the Li-ion concentration and the Li-ion mobility. Since there is little difference in the Li-ion concentration between the modified LiCoPO₄ and the unmodified LiCoPO₄, the increased Li-ion mobility is a likely hypothesis for the improvement in discharge capacity and rate capability (rate shown in FIG. 6) of Cr,Fe-substituted LiCoPO₄ relative to LiCoPO₄.

TABLE 1 Nominal composition a (Å) b (Å) c (Å) V (Å³) LiCoPO₄ 10.2048 5.9245 4.7017 284.3 LiCo_(0.9)Fe_(0.1)PO₄ 10.2175 5.9335 4.7025 285.1 Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) 10.1703 5.9204 4.6991 282.9 Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) 10.2019 5.9299 4.6976 284.2 Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) 10.2009 5.9314 4.6999 284.4 Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) 10.2060 5.9308 4.6986 284.4

When a higher Cr content was prepared, Li_(1.05)Co_(0.8)Fe_(0.10)Cr_(0.10)(PO₄)_(1.05), the XRD pattern showed an extra peak around 25 degrees two-theta which matches Li₉Cr₃P₈O₂₉. This is in agreement with the electrochemical results which showed an optimal level of 0.05 Cr, since at a higher Cr content a non-electrochemically active second phase (Li₉Cr₃P₈O₂₉) appears.

FIG. 5 compares the XRD patterns of Li_(1+y/2)Co_(0.90·y)Fe_(0.10)Ti_(y)(PO₄)_(1+y/2) where y=0.025, 0.05 and 0.10. The pattern matches LiCoPO₄ except for a broad peak at approximately 24.5 degrees two-theta which can be assigned to a LiTi₂(PO₄)₃-like phase that is present in all the Ti-containing samples. This secondary phase grows as the Ti level increases. A unit cell volume of Li_(1.025)Co_(0.85)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) was determined to be 284.4 Å³. The lattice parameters also are listed in Table 1 with comparison to LiCoPO₄ [11] and LiCo_(0.9)Fe_(0.1)PO₄ [12]. The unit cell volumes of LiCoPO₄ and LiCo_(0.9)Fe_(0.1)PO₄ are 284.3 Å³ and 285.1 Å³, respectively. The slightly smaller unit cell is consistent with the substitution of either Ti⁴⁺ (0.65 Å) or Ti³⁺ (0.67 Å) for Co²⁺ (0.745) [13].

The presence of the secondary LiTi₂(PO₄)₃-like phase suggests that the mechanism to increase the electrochemical performance of Ti,Fe-substituted LiCoPO₄ may differ from that of the Cr,Fe substituted LiCoPO₄. As previously discussed, at a 0.05 Cr substitution level, a bulk substitution for Co is obtained, In contrast, the Ti, Fe modified LiCoPO₄ samples all contain a small fraction of a LiTi₂(PO₄)₃-like phase and we therefore suggest that the improvement in electrochemical performance for Ti,Fe modified LiCoPO₄ may result from the beneficial effect this phase has on the Li-ion conductivity of the substituted LiCoPO₄.

The LiTi₂(PO₄)₃-like phase has the NASICON structure and it is known to be an excellent Li-ion conductor as a result of the structural characteristics of the NASICON structure which favor high Li-ionic conductivity, Furthermore, the interface of two Li-ion conducting materials can lead to orders of magnitude increased Li-ion conductivity on both sides of the interface through a synergistic effect.

Having shown the improvements in capacity fade because of double compositional modifications of LiCoPO₄, the favorable effects of composition modification on the capacity of Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) as a function of rate are shown in FIG. 6. Typical discharge curves at three rates, C/10, C/2 and C are plotted. The capacity at C/10 is about 132 mAh/g. At a 1 C rate the capacity is about 126 mAh/g, thus the material shows an excellent rate capability. The improvements are suggested to result from enhanced Li-ion conduction.

In addition to the above, the coulombic efficiency of the nominal composition of Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)M_(0.05)(PO₄)_(1.025) with M=Cr or Ti was improved by adding Si. The discharge capacity as a function of cycle and cycle life is shown in FIG. 7 for Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) relative to its Si free analog. As shown in the figure, a significant improvement in the coulombic efficiency and cycle life was observed. Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Ti_(0.05)(PO₄)_(1.025) showed a similar improvement in coulombic efficiency and an even greater improvement in cycle life as shown in FIG. 8.

The x-ray diffraction pattern of the nominal composition Li_(1.025)Co_(0.84)Si_(0.01)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025) is shown in FIG. 9. There is a peak around 25 degrees which does not correspond to the phospho-olivine structure. This might either be Si related or Li₉Cr₃P₈O₂₉ as previously discussed. The lattice constants (see Table 1) show a decrease in the unit cell volume relative to LiFe_(0.10)Co_(0.90)PO₄, but the unit cell volume is larger than Li_(1.025)Co_(0.85)Fe_(0.10)Cr_(0.05)(PO₄)_(1.025), and suggests that inclusion of Si in the starting composition 

We claim:
 1. A positive electrode material comprising: a Li-ion positive electrode material having a nominal stoichiometry of: Li_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) where M is a trivalent cation selected from at least one of Cr, Ti, Al, Mn, Ni, V, Sc, La and/or Ga, M′ is a covalent cation selected from at least one of Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y is within a range of 0<y≦0.10, x is within a range of 0≦x≦0.20, z is within a range of 0≦z≦0.10 and d is within a range of 0≦d≦0.20; said Li-ion positive electrode material having an initial capacity of at least 120 mAh/g and a discharge capacity of at least 100 mAh/g after 500 cycles.
 2. The positive electrode material of claim 1, wherein z=0, y is within the range of 0.02≦y≦0.08, x is within the range of 0.05≦x≦0.15 and M=Cr or Ti.
 3. The positive electrode material of claim 2, wherein y is within the range of 0.04≦y<0.06 and x is within the range of 0.08≦x≦0.12.
 4. The positive electrode material of claim 2, wherein y=0.05 and x=0.10.
 5. The positive electrode material of claim 4, wherein M=Cr and said Li-ion positive electrode material has an initial capacity of at least 125 mAh/g and a discharge capacity of at least 105 mAh/g after 500 cycles.
 6. The positive electrode material of claim 4, wherein M=Ti.
 7. The positive electrode material of claim 1, z is within a range of 0<z≦0.1.
 8. The positive electrode material of claim 7, wherein said Li-ion positive electrode material has a coulombic efficiency between 97-100% at a C/3 cycle rate.
 9. The positive electrode material of claim 8, wherein z is within the range of 0<z≦0.05.
 10. The positive electrode material of claim 9, wherein z is within the range of 0<z≦0.02.
 11. The positive electrode material of claim 10, wherein z=0.01.
 12. A high voltage lithium ion positive electrode material comprising: a Li-ion positive electrode material having a nominal stoichiometry of: Li_(1+y/2)Co_(1−x−y−z−d)Si_(z)Fe_(x)M_(y)M′_(d)(PO₄)_(1+y/2) where M is a trivalent cation selected from at least one of Cr, Ti, Al, Mn, Ni, V, Sc, La and/or Ga, M′ is a divalent cation selected from at least one of Mn, Ni, Zn, Sr, Cu, Ca and/or Mg, y is within a range of 0<y≦0.10, x is within a range of 0≦x≦0.20, z is within a range of 0≦z≦0.10 and d is within a range of 0≦d≦0.20; said Li-ion positive electrode material having a discharge capacity of at least 100 mAhg⁻¹ after 500 cycles.
 13. The positive electrode material of claim 12, wherein z=0, y is within the range of 0.02≦y≦0.08, x is within the range of 0.05≦x≦0.15 and M=Cr or Ti.
 14. The positive electrode material of claim 13, wherein y is within the range of 0.04≦y≦0.06 and x is within the range of 0.08≦x≦0.12.
 15. The positive electrode material of claim 14, wherein y=0.05 and x=0.10.
 16. The positive electrode material of claim 15, wherein M=Cr and said Li-ion positive electrode material has an initial capacity of at least 125 mAh/g and a discharge capacity of at least 105 mAh/g after 500 cycles.
 17. The positive electrode material of claim 4, wherein M=Ti.
 18. The positive electrode material of claim 12, wherein z is within a range of 0<z≦0.10; said Li-ion positive electrode material having a coulombic efficiency between 97-100% at a C/3 cycle rate.
 19. The positive electrode material of claim 19, wherein z is within the range of 0<z≦0.05.
 20. The positive electrode material of claim 19, wherein d=0 and z=0.01. 